Dipartimento di / Department of
Scienza dei MaterialiDottorato di Ricerca in / PhD program Scienza e Nanotecnologia dei Materiali Ciclo / Cycle XXXIII
SUSTAINABLE SYNTHETIC METHODOLOGIES
FOR THE PREPARATION OF ORGANIC
SEMICONDUCTING MATERIALS: ORGANIC
(OPTO)ELECTRONICS GROWING “GREEN”
Cognome / Surname: Calascibetta Nome / Name: Adiel Mauro Matricola / Registration number: 769698
Tutore / Tutor: Prof. Luca Beverina
Coordinatore / Coordinator: Prof. Marco Bernasconi
Table of Contents
Introduction ... 1
Organic Electronics en Route Toward a Sustainable Future... 1
Bibliography ... 4
Chapter 1.
Organic Semiconducting Materials for (Opto)Electronic Applications
... 5
1.1. Organic π-Conjugated Systems: Structures and Properties ... 6
1.1.1. Structural Engineering of the Energy Gap in π-Conjugated Systems ... 7
1.1.2. Control of Structures and Morphologies: Small-Molecules VS Polymers ... 11
1.1.3. Charge Transport in OSCs ... 13
1.1.4. Doping in OSCs... 15
1.1.5. Optical Properties of OSCs ... 17
1.2. Devices: “The Big Three” ...20
1.2.1. Large Area Electronics with Organic Field-Effect Transistors ... 20
1.2.1.1. Organic Materials for OFETs... 24
1.2.2. High-Performance Organic Light-Emitting Displays... 27
1.2.2.1 Organic Materials for OLEDs ... 30
1.2.3. Solution-Processed Organic Photovoltaics ... 32
1.2.3.1. Organic Materials for OPVs ... 35
Bibliography ...38
Chapter 2.
Modern Sustainable Synthetic Strategies to the Green Access of
Semiconducting π-Conjugated Molecular Structures ...42
2.1. Organic Materials: The Lab to Fab Transition ...43
2.2. Palladium-Catalysed Cross-Coupling Reactions ...45
2.3. Introducing Sustainability with Aqueous Solutions of Surfactants ...48
2.3.1. Organic Transformations in Water Through the Micellar Catalysis Approach ... 50
2.3.2. Micellar Enhanced Suzuki-Miyaura Couplings... 53
2.3.3. Organic Semiconductors Access in Water and Under Air ... 55
2.3.4. Effect of Co-Solvents on Micellar Catalysis ... 56
2.4. Aim of the Work ...61
2.5.1. Introduction... 63
2.5.2. Results and Discussion ... 64
2.5.2.1. Optimization of Surfactant Enhanced DHA Coupling on a Model Reaction ... 64
2.5.2.2. Scope and Generality of the Method in the Preparation of Conjugated Building Blocks ... 70
2.5.3. Conclusion ... 72
2.5.4. Experimental Section... 72
2.5.4.1. Materials and Instruments... 72
2.5.4.2. General Procedure for Emulsion Couplings and E-factor Calculations... 73
2.5.4.2.1. Synthesis of 2-(4-methoxyphenyl)-5-hexylthiophene (model compound, 1) ... 73 2.5.4.2.2. Synthesis of 2,3-dihydro-5,7-bis(4-methoxyphenyl)thieno[3,4 b][1,4] dioxine (8) ... 77 2.5.4.2.3. Synthesis of 2,3-dihydro-5,7-bis(4-methylnaphthyl)thieno[3,4-b][1,4]dioxine (9) ... 80 2.5.4.2.4. Synthesis of 2-(5-hexyl)-2-thienyl-[1]benzothieno[3,2-b][1]benzothiophene (10)... 83 2.5.4.2.5. Synthesis of 5,5'-bis(3-hexylphenyl)-2,2'-bithiophene (11) ... 86 2.5.4.2.6. Synthesis of 2,5-bis(3-hexylphenyl)thieno[3,2-b]thiophene (12) ... 89 2.5.4.2.7. Synthesis of 9,10-bis(5-hexyl-2-thienyl)anthracene (13) ... 92 2.5.4.2.8. Synthesis of 4,7-bis(5-hexyl-2-thienyl)benzo[c][1,2,5]thiadiazole (14)... 95 2.5.4.2.9. Synthesis of 1,4’-bis(3-hexylphenyl)-2′,2″,3′,3″,5′,5″,6′,6″-octafluorobiphenyl (15) ... 98
2.6. Micellar Suzuki-Miyaura Synthesis of Symmetrical and Unsymmetrical
Benzothiadiazole Luminescent Derivatives in Water and under Air... 101
2.6.1. Introduction...101
2.6.2. Results and Discussion ...103
2.6.2.1. Micellar Suzuki-Miyaura Synthesis of BT Derivatives... 103
2.6.2.1.1. Influence of the Surfactant Concentration... 104
2.6.2.1.2. Influence of the Reagent’s Formal Concentration ... 106
2.6.2.1.3. Influence of the Surfactant’s Nature ... 106
2.6.2.1.4. Influence of the Catalyst... 107
2.6.2.1.5. Troublesome DTBT Selectivity and Computational Analysis... 109
2.6.2.1.6. One-Pot Synthesis of Unsymmetrical Derivatives ... 111
2.6.3. Conclusion ...113
2.6.4. Experimental Section...114
2.6.4.1. Materials and Instruments... 114
2.6.4.2. Details on the GC-MS Response Factor Calibration ... 114
2.6.4.3. General Synthetic Procedure for Synthesis of Symmetrical BT Derivatives .. 115
2.6.4.3.2. Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole (17) ... 116 2.6.4.3.3. Synthesis of 4,7-di(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTBT) ... 116 2.6.4.3.4. Synthesis of 4,7-di(thiophene-3-yl)-5,6-difluoro-2,1,3-benzothiadiazole (18) ... 117 2.6.4.3.5. Synthesis of 4,7-diphenyl-5,6-difluoro-2,1,3-benzothiadiazole (19) ... 120 2.6.4.3.6. Synthesis of 4-bromo-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (TBF) ... 120
2.6.4.4. General Procedure for the One-Pot Synthesis of Unsymmetrical BT Derivatives ... 121 2.6.4.4.1. Synthesis of 4-phenyl-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (20) ... 121 2.6.4.4.2. Synthesis of 4-(2,4-difluorophenyl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (21) ... 124 2.6.4.4.3. Synthesis of 4-(4-formylphenyl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (22) ... 127 2.6.4.4.4. Synthesis of 4-(4-(trifluoromethyl)phenyl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (23) ... 130 2.6.4.4.5. Synthesis of 4-(2,5-dimethylphenyl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (24) ... 133 2.6.4.4.6. Synthesis of 4-(naphthalen-1-yl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (25) ... 136 2.6.4.4.7. Synthesis of 4-(9H-fluoren-2-yl)-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (26) ... 139 2.6.4.4.8. Synthesis of 4-[5-hexyl-(2,2-bithiophen)-5-yl]-7-(thiophene-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (27) ... 142
2.7.
Suzuki-Miyaura
Synthesis of
π-Extended
[1]BenzoThieno[3,2-b][1]BenzoThiophene (BTBT) Derivatives in an Aromatic Micellar Medium . 145
2.7.1. Introduction...1452.7.2. Results and Discussion ...146
2.7.3. Conclusion ...151
2.7.4. Experimental Section...152
2.7.4.1. Materials and Instruments... 152
2.7.4.2. Synthesis of BTBT and its Brominated Derivatives... 152
2.7.4.3. General Procedures for the Preparation of BTBT Derivatives using K -EL Emulsion ... 156 2.7.4.3.1. Synthesis of BTBT-Ph ... 156 2.7.4.3.2. Synthesis of C10-BTBT-Ph ... 157 2.7.4.3.3. Synthesis of BTBT-Th-C6 ... 157 2.7.4.3.4. Synthesis of BTBT-4Th-C6 ... 158 2.7.4.3.5. Synthesis of BTBT-(Th-C6)2 ... 161 2.7.4.3.6. Synthesis of BTBT-(4Th-C6)2 ... 161
2.8. Novel, Fully Sustainable and Cost-Effective Multi-Step Synthesis of
Spiro-OMeTAD HTM for Highly Efficient Hybrid Perovskites Solar Cells ... 166
2.8.1. Introduction...166
2.8.2. Results and Discussion ...170
2.8.2.1. One-Pot Construction of the SBF Core ... 170
2.8.2.2. Micellar Tetra-Bromination Reaction of the SBF ... 171
2.8.2.3. Tetra-Buchwald-Hartwig Amination Reaction ... 173
2.8.2.4. PSCs Characterization of Newly Synthetized and Commercially available Spiro-OMeTAD HTMs ... 177
2.8.3. Conclusions...182
2.8.4. Experimental Section...183
2.8.4.1. Materials and Instruments... 183
2.8.4.2. Synthesis of 9,9’-spirobifluorene (SBF) ... 183
2.8.4.3. Synthesis of 2,2’,7,7’-tetrabromo-9,9’-spirobifluorene (SBF-4Br) ... 184
2.8.4.4. Synthesis of 2,2’,7,7’-tetrakis(N,N-bis-4-methoxyphenylamine)-9,9’-spirobifluore (spiro-OMeTAD) ... 184
Chapter 3.
Extension of Sustainable Approaches to the Synthesis of π-Conjugated
Polymers ... 196
3.1. Synthesis of PF8T2 and PF8BT Conjugated Polymers by Sustainable
Suzuki-Miyaura Polycondensation in Water and under Air ... 198
3.1.1. Introduction...198
3.1.2. Results and Discussion ...199
3.1.2.1. Optimization of Formulative Conditions on a Model Small Molecule Compound... 199
3.1.2.2. Surfactant Promoted S-M Polymerization Reactions to Sustainable Access PF8T2 and PF8BT ... 201
3.1.2.3. Evaluation of the Emission Efficiency and Photostability of Emulsion vs Homogeneous Phase Polymerized Materials... 204
3.1.3. Conclusions...208
3.1.4. Experimental Section...209
3.1.4.2. General Synthetic Procedure for the Synthesis of the Model Compound 2,7
-dithienyl-9,9-dioctylfluorene (43) ... 209
3.1.4.3. General Synthetic Procedure for the Polymerizations... 210
3.1.4.3.1. Synthesis of PF8T2 Control Sample ... 210
3.1.4.2.2. Synthesis of PF8BT Control Sample ... 211
3.1.4.2.3. General Synthetic Procedure for the Optimization of the Polymerization Method in Micellar and Emulsion Environment... 212
3.2. Synthesis of 3D Conjugated Porous Polymers (CPPs) by Sustainable
Suzuki-Miyaura Polycondensation in Water and under Air ... 214
3.2.1. Introduction...214
3.2.2. Results and Discussion ...215
3.2.2.1 Synthesis of the Branch-Core Monomers ... 215
3.2.2.2 Emulsion S-M Polycondensations to Access CPPs... 219
3.2.3. Conclusions...223
3.2.4. Experimental Section...223
3.2.4.1. Materials and Instruments... 223
3.2.4.2. Synthesis of 2,2’,7,7’-tetrakis(4,4,5,5-tetramethyl-1,3,2 dioxaborolan)-9,9’-spirobifluorene (SBF-4Bpin) ... 224
3.2.4.3. Synthesis of 2,2′,7,7′-tetrakis(thiophene-2yl)-9,9′-spirobifluorene (SBF-4Th)224 3.2.4.4. Synthesis of 2,2′,7,7′-tetrakis(5-bromo-thiophene-2yl)-9,9′-spirobifluorene (SBF-4Th-4Br) ... 225
3.2.4.5. Synthesis of PSBFT2 Control Sample in Standard Homogeneous Phase Conditions ... 225
3.2.4.6. Synthesis of PSBFBT Control Sample in Standard Homogeneous Phase Conditions ... 228
3.2.4.7. General Synthetic Procedure for the Emulsion Polymerizations ... 230
3.2.4.7.1. Characterization of PSBFT2 in Emulsion Conditions ... 231
3.2.4.7.2. Characterization of PSBFBT in Emulsion Conditions ... 233
3.2.4.7.3. Characterization of PSBFBT-Inv in Emulsion Conditions ... 236
3.2.4.7.4. Characterization of PSBF2FBT in Emulsion Conditions... 238
3.2.4.7.5. Characterization of PSBFDTBT in Emulsion Conditions... 241
3.2.4.7.6. Characterization of PSBFDT2FBT in Emulsion Conditions ... 243
References ... 245
Introduction
Organic Electronics en Route Toward a
Sustainable Future
We live immersed in an electronic world where it becomes increasingly difficult to imagine a single day without the assistance of modern technology. Whether we talk about electronic circuits, processors, sensors, lasers, memory elements, displays, photodiodes, solar cells and so forth, those devices are indispensable tools of our daily routine. Electronics, and more specifically integrated circuits (IC), have dramatically changed our lives and the way we interact with the world. Economic, health, and national security rely on and are positively impacted by electronic technology. The commercial success of integrated electronics is based on a mutual development of technology and applications, where technical progress and economic growth support each other. The impressive technological achievements of our time are the result of more than fifty years of ongoing electronics revolution, where inorganic semiconductors with their archetypical examples silicon and gallium arsenide remain fundamental. Nevertheless, modern silicon microelectronics and its manufacturing technology are characterized by extremely expensive and complicated processes; they generally require high-temperature and vacuum-based manufactures limiting the technology throughput. Even if novel electronic products become more and more energy efficient in application, it is not their power consumption that creates an energy imbalance, but actually the energy expended in their production phase and stored (embodied) in their inner constituents, i.e. microchips, processors, displays, etc.1,2 The higher the demand of the market, the more versatile the offer, and the higher the
energy consumption expended in the manufacturing processes. Modern electronic technology has turned the relationship energy consumed during fabrication vs. energy consumed during exploitation of the product to a complete imbalance: just think that a simple laptop or a smart phone contains more embodied energy than a 1980s or 1990s edition automobile.3 The manufacturing process of a
significant amount of a high-quality inorganic semiconductor or other nanomaterial of any modern electronic gadget requires up to six orders of magnitude or more energy than the energy required for processing a plastic or a metal component.1 The colossal demand of electronics is leading not only to
an energy imbalance, but also to a series of unfortunate and undesirable consequences: (i) a massive amount of wastes, including both electrical and electronic equipment (WEEE) and manufacturing refuses, besides (ii) a rapid exhaustion of already scarce natural elements, such as gallium (annual production of ~215 tons) and indium (annual production of ~1100 tons including recycling) both of which have an estimated availability of about 20 years until they will run out completely.4 The
resources and methodologies used to manufacture electronic devices raise urgent questions about the negative environmental impacts of the production, use, and disposal of electronic devices. A paradigm shift of the way we control both the resource exploitation, and the electronics manufacturing/disposal is required in order to minimize the negative impact of our present and future generations on the environment and to create a sustainable future. As defined by the United Nations
World Commission on Environment and Development, a sustainable development is established when
humanity ensures its present needs without compromising the ability of future generations to meet their own needs.5
introducing “eco-friendly” device platforms, but also using more “eco-friendly” manufacturing methods to do so. In fact, sustainability cuts across the entire life cycle of an electronic product, from raw resources to disposal. In this regard, the use of organic materials to construct electronic devices offers a more eco-friendly and cost-affordable approach to grow our electronic world.6 Organic-based
technologies consisting of “soft” conjugated small molecules and polymers as the core semiconductor element are meant to address the energy, cost-inefficiency and sustainability issues posed by their inorganic counterparts. Organic electronics (OE) entered the research field in 1977 with the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger,7 opening up new fields
of science and research of huge interest. The progress in those fields has grown exponentially and nowadays π-conjugated organic structures, called organic semiconductors (OSCs), can be successfully implemented in several electronic devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), sensors, integrated circuits, solar energy storage, memories, organic photovoltaic cells (OPVs), RF-ID tags etc.8–12 The growing list of applications
reflects the complexity of the topic and the wide possible uses for organic electronics, and it is likely that the list will even grow in the future. The first examples, such as displays in mobile appliances, have already found their way to market as replacements for traditional components in existing products. Today, OLEDs are widely used in mobile phones and it is possible to buy printed organic photovoltaic modules and simple printed memories. In particular, OE distinguishes itself from traditional electronics because it is possible to explore a limitless library of materials through structural design and functionalization at molecular level, as well as process active materials from solution and form functional films to realize light displays and circuits that are completely flexible. Furthermore, owing to the key features of low-cost, low-temperature and processability from solution over arbitrary substrates, OSCs are amenable to inexpensive, fast, and high-throughput manufacturing processes; for example, they are compatible with all-in-line roll-to-roll printing techniques to fulfil the constantly increasing demand for large-area electronics.13 Finally, biocompatible and
biodegradable electronic devices may be realized only with organic materials, which have the potential to interface with biological systems in a way not possible with today’s silicon-based electronics. This opens a vast world of possibility with respect to medical, sensing, and other human interface applications.
Despite intense efforts of the scientific community during the past 30 years, the performance14–16 and
stability17 of organic semiconductors remain at current times major hurdles in their development as
solid competitors of the inorganic counterparts. As a consequence, the large-scale immediate replacement of hard core inorganic components with organic counterparts is not immediately foreseen.14–18 Nevertheless, the versatile nature of OE combined with the promise the field holds forth
for environmental and social sustainability point the way to a very long-lived set of technologies. It is not just the devices themselves that promise to be more eco-friendly (ending with potentially biodegradable or recyclable devices) than silicon-based electronics, but also their manufacture processes. With OLEDs already making a large footprint in the market of flat panel displays and with OFETs and OPVs entering small scale commercial production in e-books and roll-to-roll fabricated photovoltaics, the research field of OE is mature enough to focus on achieving ambitious goals of generating “green” avenues for a sustainable electronic future.
As already discussed, OE is a platform technology that enables multiple applications, which vary widely in their specifications. Since the technology is still in its early stage but aiming to a transition from lab-to-fab scale of production, it is important to develop a common opinion about what kind of products, processes and materials will be available and when. “Green” chemistry and technologies are carving avenues towards achieving the ambitious goal of sustainability in the field of electronics,19
complex biological functions (e.g. transduction, sensing, recognition, event triggering, etc.) as a tool for interfacing electronics with various forms of life. Since green materials and technologies are now in the stage of emerging concepts, offering at this time a clear definition of “greenness” is not unequivocal; that is because achieving industrial green synthesis of green organic materials by means of green technologies remains truly challenging at this time. For example, many benign dyes and pigments commonly employed in textile, cosmetic, and food coloring industries are produced through industrially attractive, low-cost and high throughput synthesis;20 nevertheless, in many cases their
manufacture involves and generates environmentally unfriendly by-products and waste. However, the field of OE is in its infancy with respect to class of devices on the market. Realizing the vision of OE as a more innovative, accessible, and sustainable approach to grow our electronic world will require overcoming key research challenges for advancing the field in a way that will maximize its potential positive impact on society.
In their quest to achieve electronics sustainability by solving the above-mentioned energy deficiency puzzle and redressing the unfolding environmental disaster, scientists are often inspired both by the apparent simplicity and by the true complexity of nature. Nature is an extremely efficient energy consumption engine that we could use for infinite inspirations.
Bibliography
1. Gutowski, T. G. et al. Thermodynamic analysis of resources used in manufacturing processes. Environ. Sci. Technol. 43, 1584–1590 (2009).
2. Allwood, J. M., Ashby, M. F., Gutowski, T. G. & Worrell, E. Material efficiency: A white paper. Resour. Conserv. Recycl. 55, 362–381 (2011).
3. Williams, E. D. Revisiting energy used to manufacture a desktop computer: Hybrid analysis combining process and economic input-output methods. IEEE Int. Symp. Electron. Environ. 80–85 (2004) doi:10.1109/isee.2004.1299692.
4. Zoeteman, B. C. J., Krikke, H. R. & Venselaar, J. Handling WEEE waste Hows: On the effectiveness of producer responsibility in a globalizing world. Int. J. Adv. Manuf. Technol. 47, 415–436 (2010).
5. Chatterjee, D. K. World Commission on Environment and Development. Encycl. Glob.
Justice 1163–1163 (2011) doi:10.1007/978-1-4020-9160-5_1126.
6. Irimia-Vladu, M. ‘Green’ electronics: Biodegradable and biocompatible materials and devices for sustainable future. Chem. Soc. Rev. 43, 588–610 (2014).
7. Chiang, C. K. et al. Electrical conductivity in doped polyacetylene. Phys. Rev. Lett. 39, 1098–1101 (1977).
8. Guo, X., Baumgarten, M. & Müllen, K. Designing π-conjugated polymers for organic electronics. Prog. Polym. Sci. 38, 1832–1908 (2013).
9. Dou, L., Liu, Y., Hong, Z., Li, G. & Yang, Y. Low-Bandgap Near-IR Conjugated Polymers/Molecules for Organic Electronics. Chem. Rev. 115, 12633–12665 (2015). 10. Groves, C. Organic light-emitting diodes: Bright design. Nat. Mater. 12, 597–598 (2013). 11. Wang, C., Dong, H., Hu, W., Liu, Y. & Zhu, D. Semiconducting π-conjugated systems in field-effect transistors: A material odyssey of organic electronics. Chem. Rev. 112, 2208– 2267 (2012).
12. Facchetti, A. π-Conjugated polymers for organic electronics and photovoltaic cell applications. Chem. Mater. 23, 733–758 (2011).
13. Zvezdin, A., Di Mauro, E., Rho, D., Santato, C. & Khalil, M. En route toward sustainable organic electronics. MRS Energy Sustain. 7, 3–10 (2020).
14. Ma, H., Acton, O., Hutchins, D. O., Cernetic, N. & Jen, A. K. Y. Multifunctional phosphonic acid self-assembled monolayers on metal oxides as dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic field-effect transistors. Phys.
Chem. Chem. Phys. 14, 14110–14126 (2012).
15. Mei, J., Diao, Y., Appleton, A. L., Fang, L. & Bao, Z. Integrated materials design of organic semiconductors for field-effect transistors. J. Am. Chem. Soc. 135, 6724–6746 (2013). 16. Janssen, R. A. J. & Nelson, J. Factors limiting device efficiency in organic photovoltaics.
Adv. Mater. 25, 1847–1858 (2013).
17. Jørgensen, M. et al. Stability of polymer solar cells. Adv. Mater. 24, 580–612 (2012). 18. Ante, F. et al. Contact resistance and megahertz operation of aggressively scaled organic
transistors. Small 8, 73–79 (2012).
19. Lee, E. K., Lee, M. Y., Park, C. H., Lee, H. R. & Oh, J. H. Toward Environmentally Robust Organic Electronics: Approaches and Applications. Adv. Mater. 29, 1–29 (2017).
Chapter 1
Organic Semiconducting Materials for
(Opto)Electronic Applications
Photoactive and electroactive organic systems, known as organic semiconductors (OSCs), have been playing a crucial role as functional materials in the development of organic electronic, photonic and optoelectronic devices mainly concerned with thin-films and flexible architectures.1,2 Among these
technologies organic photovoltaic devices (OPVs), organic light-emitting diodes (OLEDs) and organic thin-film transistors (OTFTs) have been the central topics of current research and development.3–5 OPVs have been receiving a great deal of interest as promising candidates for
next-generation solar cells helping facing current global issues of non-abundant materials, environment and energy. OLEDs have already found their way to market with the successful practical application as portable flat-panel displays in smartphones and televisions and are expected to accomplish next-generation solid-state lighting. OTFTs are key components for driving various devices to a cheap and practical fabrication of thin-film, light-weight and flexible architectures by use of printing techniques.
OSCs consist of both small-molecular and polymeric materials and they are generally characterized by low-cost, lightweight, wide availability of raw materials, synthetic flexibility, low production and energy costs, easy processibility, and infinite possibilities of chemical engineering and functionalization for tailoring physical properties to achieve specific functions.6 Actually, OSCs are
expected to be key materials for 21st-century industries: they can be solution processed, low-temperature evaporated or sublimed (at relatively low vacuum levels), self‐assembled and formulated in functional inks which are compatible with flexible plastic substrates. These advantages allow to deploy organic-based devices in large-area through low cost roll-to-roll printing or coating processes, profiting from well-established know-how about printing processes, papers, and photographic films production for example.7 Digital printing (such as inkjet) makes it even possible to produce
custom-designed plastic electronics on a massive scale by remarkably cost-effective continuous reel-to reel processes.
The science of organic and printed (opto)electronicsinclude wide areas, from the structural design and synthesis of photoactive and electroactive materials, to the elucidation of their physical and chemical properties as well as the structures, fabrication, and performance evaluation of the devices, and the creation of new knowledge derived from the operation of such devices. Overall, the technology has already entered in a small market reality with first relative ly simple products,10
however further growth could be reached in the near future depending on progress in materials, processes, devices, circuit designs and environmental impact.
1.1. Organic π-Conjugated Systems: Structures and
Properties
Organic semiconductors are π-electron systems with the capability to absorb and emit light in the wavelength region from ultraviolet to near infrared, generate and transport charge carriers and often exhibit excellent nonlinear optical properties. These materials possess a π -conjugated backbone (generally represented by the alternation of single and double bonds) which can be simplified to a system of linked diene units leading to extended p-orbitals that, by resonance effect, confer electron delocalization and the ability to transport charge. The conjugation extension and functionalization of this backbone have a dramatic impact on optical and electrical properties of the material. Conversely, non-conjugated structures such as polyethylene or teflon are excellent insulators. The solid -state structure of conjugated systems is dominated by individual molecular units bound together by weak interactions, principally Van der Waals forces such as dipole-dipole and π-π stacking, which impart unique electronic properties depending on the packing structure.8 These should be contrasted with the
far stronger intramolecular covalent interactions, which nevertheless allow for soft bonded degrees of freedom, notably torsional motion along the backbone and side chains.9 As a result, structure
packing is governed by both energy and entropy, giving rise to complex morphologies that incorporate amorphous, liquid-crystalline, and crystalline ordering.
The description of optical and electrical properties of organic semiconductors is gen erally performed by considering the energy and electronic distribution of the corresponding frontier molecular orbitals (FMOs): the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Considering a given structure, as its conjugation length increases the HOMO-LUMO energy gap decreases, complemented by the reduction in distan ce between bond and anti-bond molecular orbitals until they are so close to become indiscernible from each other ( Figure 1). This leads to the formation of two bands: the valence band where the electrons are located, and the conduction band, where the electrons can jump as soon as they receive enough energy to overcome the energy gap. The limit of this representation is reached when the gap betwee n HOMO and LUMO is so small that the two orbitals basically overlap, as in the case of a metal. Organic materials are identified as semiconductors an energy gap (EG) is always present, even in the case of Polyacetylene
1.1.1. Structural Engineering of the Energy Gap in
π-Conjugated Systems
As previously discussed, the energy gapEG between the HOMO-LUMO levels of a specific material
decreases as a negative function of the extent of conjugation of the structure. However, this trend is not linear, but it tends asymptotically to a set value given by several energy contributions. This concept was formalized by Roncali11 in the following equation:
EG = Ebla + ERes + Esub + Eθ + EInt
These contributions can be represented by five parameters:
• The bond-length alternation energy (Ebla), which usually represent the major contribution and
depends on the difference between single and double bond lengths of the π-conjugated core. This parameter is particularly pronounced in aromatic systems, where the two limiting mesomeric forms aromatic and quinoid (Figure 2) obtained by the flip of the double bonds are not energetically equivalent.
• The aromatic stabilisation resonance energy (ERes) of cyclic π-systems.There is a competition
between π-electron confinement within the aromatic rings and delocalization along the whole conjugated chain. The energy needed to switch from the aromatic to the quinoid form directly depends on ERes of a given aromatic unit. Typically, highly delocalized π-electrons are
essential to achieve optimal electronic properties.
• The electronic effect of substituents (Esub). The introduction of electron-withdrawing or
electron-releasing substituents strongly affect the HOMO-LUMO levels.
• The planarity of the molecule (Eθ). The degree of interannular rotations relates to different
conformations of the conjugated core. Any distortion from backbone planarity due to interannular rotations about single bonds will increase EG because the orbital overlap varies
with the cosine of the torsional angle θ.
• The solid-state interactions between individual molecules (EInt). Intermolecular interactions
can influence the properties of a single conjugated molecule by tuning EG when assembling.
Since (opto)electronic devices require active materials with a specific combination of properties (for instance specific visible light interactions and efficient charge transport) the engineering of the en ergy gap of π-conjugated structures is the obvious solution to obtain materials with tailored physical properties. The molecular architecture useful to construct functional materials is related to one or several of these contributions. First, energy gap restriction only through conjugation length extension is a restricted strategy since at some point the effective conjugation length becomes saturated and the energy gap begins to level off.12,13The following are synthetic principles useful to module and in
particular to narrow the energy gap of semiconducting materials in order to control (opto)electronic properties.
Resonance energy reduction
The contribution given by Ebla explains why conjugated aromatic rings are highly employed to
achieve suitable (opto)electronic properties. Thiophenes are one of the most essential classes of aromatic heterocyclic compounds in the field of materials chemistry. Thiophene -based materials are beneficial due to their semiconducting nature, nonlinear optical responses and effective electron transport properties.14 Considering the contribution of E
Res in the EG tuning, the choice of less
aromatic thiophene rings over benzene rings and the insertion of double bonds between the conjugated aromatic rings represents a simple and straightforward way to reduce EG and improve
charge delocalization. This structural design include two well-studied families of organic semiconductors: poly(p-phenylenevinylenes) (PPVs)15 and poly(thienylenevinylenes) (PTVs)16. In
particular,the insertion of double bonds leads to a decrease of the overall aromaticity of the system and thus to a reduction of the gap. Moreover, in both cases the ethylenic linkages eliminate the rotational freedom around the ring-ring single bonds and lead to a more planar geometry (favouring also Eθ) and thus further helps in reducing EG. These effects can be evidenced by comparing the λmax
of absorbance of conjugated oligomers reported in Figure 3 containing a constant number of π-electrons in different combinations of thiophene and ethylene units.17
Figure 3. Chemical structures of oligomers 1–3 showing the effect on EG (λmax of absorbance) by adding
double bonds between the thiophene units in the molecule. Reported with permission from ref .11; © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
While the insertion of ethylenic linkages between the thiophene rings generates the expected decrease of EG, as shown by a 50 nm red shift of λmax between 1 and 2, insertion of two adjacent double bonds
counterbalanced by an increase of the vibrational freedom of the system, in which small changes in bond length alternation become possible.
Furthermore, the use of conjugated systems with an enhanced quinoid ch aracter is another popular approach for energy-gap restriction. Considering a conjugated polythiophene structure, the most immediate way to increase the quinoid character of the neutral state includes the fusion of the thiophene ring with an aromatic system with a higher resonance energy (ERes). Since the aromatic
character tends to localize in the system of highest ERes, it follows that the thiophene ring tends to
dearomatize to adopt a quinoid structure. Therefore, comparison of the ERes values for thiophene and
benzene shows that the [c]fusion of the two rings should contribute to confer a quinoid character on the thiophene system (Figure 4).18
Figure 4. Left: comparison of the ERes values for thiophene and benzene. The [c]fusion of the two rings contribute to confer quinoid character on the thiophene system, that in this case correspond to poly(benzo[c]thiophene). Right: examples of chemical structu res of other typical low-band-gap polymers based on fused ring systems.Reported with permission from ref 11;© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The electronic effect of substituents
Modulation of FMO levels and EG can be done by introducing releasing (ERG) or
electron-withdrawing (EWG) groups as described for Esub. Theintroduction of ERGs to a conjugated structure
decreases the oxidation potential of the system raising the HOMO level and reducing the EG, while
introducing EWGs increase the electron affinity of the system reducing both HOMO and LUMO levels as well as reducing the EG.11 The impact of the substituent on FMO levels depends on its
electron-releasing/withdrawing strength. Alkoxy, alkylsulfanyl or amino groups are very popular releasing substituents, while nitro, carboxy or cyano groups are very common electron-withdrawing substituents (Figure 5). The introduction of alkyl chains of sufficient length (typically 6–9 carbons) is crucial to confer solubility to highly conjugated materials in common organic solvents. Moreover, alkyl chains have a slightly inductive effect in raising its HOMO level and indirectly contribute to reduce the EG by enhancing the long-range order in the structure, through
lipophilic interactions between the alkyl chains (EInt);19 this effect is remarkably important in
regioregular polymers.20,21
Alternation of Donor-Acceptor units
Further energy gap engineering can be performed though the “Donor-Acceptor” (D-A) approach. The alternation of electron rich (donor D) and electron poor (acceptor A) units along the π-conjugated structureleads to a molecular orbital hybridization broadening the HOMO-LUMO levels and thus reducing the EG (Figure 6). This strategy introduces push-pull driving forces to facilitate electron
delocalization within the conjugated structure and leads to materials with tailored FMO levels and EG
suitable for various optoelectronic applications depending on the d onor and acceptor relative strength of the corresponding building blocks.22
Figure 6. Left: diagram showing reduced EG in donor-acceptor (D-A) structures via molecular orbital
hybridization. Right: representative D and A π-electronic units.
Rigidification of the Conjugated System
Controlling the structural conformation of conjugated materials in order to achieve high planarity is another strategy useful to finetune FMOs and reduce EG as described for Eθ. An increase in the
planarization of the π-conjugated framework allows a more efficient intermolecular π-π orbital overlap (rising the contribution of EInt), π-electrons delocalization and charge transport.23 Three
general methods can be employed to achieve highly planar conjugated conformations (Figure 7). The first involves the increase of the number of fused aromatic rings in the structure, e.g. acenes (A); the second consist of connect the neighboring aromatic rings via covalent bonds to restrict the rotation about single bonds (B); and the third requires noncovalent through-space intramolecular interactions to enhance the planarity and rigidity of conjugated backbones, implementing O···S, N···S, X···S (where X = halide), and hydrogen-bonding interactions (C).24
Structural analysis of the systems with the smallest band gaps reported so far shows that their design implicitly or explicitly leads to various combinations of some of the just discussed synthetic tools. Yamashita25 and co-workers synthesized a series of tricyclic systems involving a median proquinoid
acceptor groups such as dithienylthienopyrazine (i), dithienylthienothiadiazole (ii), and dithienylthiadiazolothienopyrazine (iii) (Figure 8) and showed that the corresponding polymers obtained exhibit EG values as low as 0.50 eV for iii for example.
Figure 8. Chemical structures of example tricylcic repeat units with a median proquinod acceptor group. Reported with permission from ref .11; © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
These tricyclic systemsrepresent a synergistic combination of alternant D-A groups, quinoidization of the polythiophene backbone by the proquinoid median groups and rigidification by intramolecular sulfur-nitrogen interactions. Even lower EG values can be obtained replacing the thiophene unit with
EDOT.26The results can be attributed to the stronger donor strength of EDOT, the additive
sulfur-nitrogen and sulfur-oxygen noncovalent interactions and to the regular 1:1 alternation of th e D-A groups.
To date, numerous small-molecule and polymeric π-conjugated organic semiconductors having diverse electronic and conformational structures have been realized, with the goals of manipulating device-specific optoelectronic properties by tuning EG, controlling the solid-state packing, solution
processability, and the resulting thin-film morphology and microstructure. The choice between molecular or polymeric semiconductors generally depends on the device to be realized, the performance required and the type of process that it is meant to be employed for its manufacture.
1.1.2. Control of Structures and Morphologies:
Small-Molecules VS Polymers
Careful control over the structures and morphologies of materials is of vital importance to ensure high performance in device operation.27 In this context, it is important to consider that processing,
properties and functions of π-conjugated polymers are quite different from the corresponding small molecular units. Conjugated materials exist as either crystals, amorphous glasses, or mesophase structures such as liquid crystals and plastic crystals. Small-molecular semiconductors usually exist as crystals and generally they are not able to form smooth films. On the other hand, conjugated polymers usually contain both crystalline and amorphous phases, ranging from highly crystalline polymers to fully amorphous polymers, and usually they can form uniform films. Examples of the representative classes of crystalline molecular materials are given by p olycondensed aromatic hydrocarbons, such as anthracene and pentacene (1); metal and metal-free phthalocyanines (2,3); porphyrines (3); fullerenes and their derivatives (4); perylenebisdiimides, such as PTCBI (5); and oligothiophenes with well-defined structures (6), among other substances (Figure 9, on the left in the blue square).28 Furthermore, it is noteworthy that certain kinds of small molecules exhibit
polymorphism (e.g., CuPc (2), TiOPc (3), and perylene pigments (5)) thus packing into crystal structures with different properties.29 In the late 1980s, research in small organic conjugated
molecules that readily form stable, amorphous glasses above room temperature bec ame very popular for OLEDs applications.30 Like polymers, amorphous conjugated molecular materials form stable,
does form smooth, uniform, amorphous films. Amorphous molecular materials have nonplanar molecular structures and take different conformers. The incorporation of bulky and heavy substituents and the enlargement of molecular size make glass formation easier and enhance the stability of the glassy state. The introduction of structurally rigid moieties such as biphenyl, terphenyl, carbazole, and truxene increases the Tg.31 Representative classes of amorphous molecular materials include
π-electron starburst molecules, such as the TDATA family (7), end -capped systems with the dimesitylboryl group such as TMB-TB (8), tetraphenylmethane derivatives (9), the family of spiro-linked molecules (10) and systems containing phosphorus atoms such as POAPF (11) (Figure 9, on the right in the orange square).28
Figure 9. Chemical structures of representative classes of crystalline molecular materials (in blue, left) and amorphous molecular materials (in red, right). Adapted with permission from ref. 28.
On the other hand, linear π-conjugated polymers mostly consist of a crystalline phase and generally they are hard to solubilize. The representative classes of linear π-conjugated polymers are given by polyacetyrene (12), poly(p-phenylene)s (13), poly(p-phenylene vinylene)s (14, PPV), poly(9,9-dialkylfluorene)s (15), polythiophenes (16), polypyrroles (17), polyanilines (18), among other substances (Figure 10).
Figure 10. Chemical structures of representative classes of linear π-conjugated polymers. Reported with permission from ref. 28.
As already discussed, to make π-conjugated polymers soluble in organic solvents, alkyl groups are often introduced as substituents. The improved synthetic methods have enabled the synthesis of highly regioregular poly(3-alkylthiophene)s with high degrees of the head-to-tail (HT) structure.32
They exhibited superior charge-carrier mobility and electric conductivity after doping relative to those of corresponding regiorandom polymers.
Generally, polymers readily form large-area, uniform films with mechanical strength and low surface roughness by solution processing. This film-forming ability of polymers makes them well suited to a number of practical applications. In order to increase morphological control and reduce batch -to-batch variability, various processing techniques have been developed. Solution -based approaches33
smallmolecular semiconductors, where vapourphase deposition enables the fabrication of high -purity thin films with good control over thickness and chemical composition.
Summarizing, considering both conjugated small molecules and polymers, each possesses their own advantages and disadvantages, also depending on the application to be accomplished. For example, small molecules are generally easier to purify and allow high-vacuum based processing, enabling the fabrication of defined multilayer devices of high performance. High-vacuum fabrication of electronic devices on the other hand is slow and costly and offers limited scalability towards bigger substrates. Moreover, small-molecular crystalline materials might prevent smooth, uniform thin-film formation, which can cause damage to devices. On the other hand, grain boundary -free amorphous materials, including both molecular materials and polymers, allow smooth, uniform amorphous thin -film formation. In addition, solution-processable semiconducting polymers are of good flexibility and consistent with numerous printing techniques for fast, cost-effective, low-temperature and large-area fabrication such as plastic circuits and electronic papers. However, a major problem that restrains the development of polymer-based devices (in addition to the limited purification techniques that can be employed for polymers syntheses) concerns the low mobilities generally observed for polymeric materials, which are several orders of magnitude lower th an that of organic small molecules. In general, either crystalline or amorphous materials are used, and the choice of which one depends on the kind of device to be realized. While polycrystalline materials have been widely used in OPVs and OFETs because of their higher charge carrier mobilities than those of amorphous materials, amorphous molecular materials have been applied successfully in OLEDs and solution -processed bulk-heterojunction OPVs.
1.1.3. Charge Transport in OSCs
The basic principle underlying charge transport in organic semiconductors is strictly related to both charge delocalization and solid-state packing. As already discussed, the solid-state structure of OSCs is built from individual molecular or polymeric units assembled by weak interactions, principally van der Waals forces (aromatic π-π stacking, dipole-dipole interactions, hydrogen bonds etc.). Thus, the electronic interaction between its individual units is generally low and the electronic structure of the individual constituents dictates the solid-state architecture. While charge carriers in inorganic semiconductors are generally delocalized, on the other hand charge carriers in organic semiconductors are generally rather localized on distinct sites (e.g. single molecules portions or small coherent chain segments for polymers).35 As such, the HOMO and LUMO levels of the conjugated
material are often used to discuss the behaviour of the solid itself. Charge carriers can be injected electrically or generated photonically (forming excitons) and they can diffuse through the system via hopping charge transport mechanism.36 .The hopping mechanism involves the transport of the charge
through a “jumping” process of charge carriers between localized states (HOMO or LUMO) of adjacent molecules/polymeric chains within the lattice. Hopping can happen due to the electronic coupling driven by weak intermolecular interactions which mediate the transfer above FMOs overlap of neighbours in π-conjugated systems. During the process, the single localized charge moves accompanied by the polarization created from the distortion of the electron density, the reorientation of the dipoles of the adjacent molecules and lattice and molecular vibrations (phonons). The effect of localization is that the mean free path of charges is typically of the order of the spacing between adjacent molecular sites, and charges moving through the disordered lattice are scattered at each molecular site.37 If localization is dominated by molecular conformational changes, then charge
transfer is thermally activated with an activation energy that is dependent on the active molecular deformations. These models are known as polaron models.
generally electron-poor molecules or polymers having high electron affinity (EA) and with high tendency upon reduction as their LUMO is quite low in energy. In these materials it is possible to inject electrons (e-) which are the species responsible of the transport of charge. Other type of OSCs
are called “ambipolar” since these materials are both p- and n- type and they can generate both charge carriers h+ and e-.37 Considering the hopping mechanism, once a p-type material oxidizes it loses an
electron from the HOMO and a radical cation specie is formed. This specie can receive an electron from an adjacent neutral molecule, which in turn becomes a radical cation. More specifically, the electron is transferred from the HOMO of the closed neutral molecule to the HOMO of the radical cation specie, causing the movement of a hole charge-carrier associated with the radical cation. On the other way around, upon reduction of an n-type material a radical anion is formed, and an electron can be transferred from the LUMO of the radical anion specie to the LUMO of an adjacent neutral molecule favouring the transport of charge (Figure 11) along the material.38
Figure 11. Top: hopping charge transport through the material’s FMO energy levels. Bottom: illustration of P-type and N-type materials structures. Reported with permission from ref. 38;© Elsevier Ltd 2007.
The charge carrier mobility is an important electronic feature of semiconductors and metals as it defines the quality of charge carrier transport of a given material. The mobility (µ) relates the drift velocity of free carriers, h+ or e-, to the driving force of an applied electric field. The charge carrier
maximum current through the device (at a given voltage) and the maximum achievable switching speed.38 There are in principle two methods to determine this crucial parameter. Either the charge
carrier mobility can be measured for a known at a given applied field (e.g. in a transit time experiment) or the current at known field and charge carrier density is recorded (e.g. in a transistor).36 OSCs are
often highly disordered (both spatially and energetically) especially in the amorphous state, and hence, their charge-carrier mobility is smaller than their covalently-bonded, highly-ordered crystalline inorganic counterparts such as C-Si. The main factors influencing the performance of devices can be divided into two categories: factors related to device physics and factors related to the semiconducting materials.The factors related to device physics involve the device configuration, the quality of the semiconductor/dielectric or semiconductor/electrode interfaces, film morphology and deposition conditions.On the other hand, factors associated to the semiconductor material relate to: (i) the band structure and FMOs’ energies, which should have enough molecular orbital overlap to ensure that the charge transferring between adjacent molecules does not have to overcome a high energy barrier; (ii) the match between the energy levels of the material and the electrodes to facilitate charge injection; (iii) the solid-state packing structures should form compact, orderly stacking, which facilitates the charge transport between molecules; (iv) the electrical, optical and chemical stability (v) the molecular weights and def ects for polymeric materials and (vi) the degree of purity. Generally, OSCs exhibit poor mobility and stability and therefore for now they can only find use in niche applications where high performances are not required. However, doping techniques have been highly exploited in order to improve OSCs conductivity and anyway, OSCs are remarkably interesting also for their excellent optical properties.
1.1.4. Doping in OSCs
Doping is a popular technique widely used to enhance conductivity of both organic and inorganic semiconductors. The introduction of defects by doping in organic semiconductors leads to the insertion of additional energy levels between the HOMO and LUMO of the material and thus reduces its energy gap. Careful control on dopant concentration allows to finetune the conductivity of the material. Doping in organic semiconductors consist in the addition of specific molecular species able to induce the formation of mobile charge carriers along the conjugated structure. In a nutshell, the doping process is a redox reaction. Depending on the conducting channel of the material (p-type or n-type) the dopant corresponds to a highly electron donating or accepting specie able to promote charge transfer in a more efficient way thanks to the right match of f rontier energy levels.40 A p-type
dopant can be figured as a strong electron accepting agent which oxidizes the p -semiconducting material by removing an electron from its HOMO and so introducing a positive hole in the structure. On the other hand, a type dopant is a strongly electron donating agent which reduces the n-semiconducting material by adding an electron to its LUMO and so introducing a negative charge in the structure.41 The integer charge transfer model and the molecular orbital levels involved in the
doping processes (p-type and n-type) are schematically represented in Figure 12.
Some of the earliest examples of conducting organic materials come from the p -type doping of polyacetylene with halogens.42 In these early conducting polymers, dopants were introduced via
exposure to halogen vapours leading to diffusion problems during processing. A more p opular and practical approach is the use of small molecules as dopants. P-type doping has been widely investigated with strong Lewis acids being employed.43 N-doping of materials is much more difficult
to achieve since the HOMO of the dopant must be high enough to allow it to donate electrons to the LUMO of various materials.N-doping has previously been achieved using alkali metals, mostly lithium,44 and molecules with extremely high HOMO levels such as tetrathianaphthacene (TTN,
EHOMO=−4.7 eV). Anyway, similar energies in the HOMO of the dopant and LUMO of the accepting
material are essential to achieve efficient results.45
Considering the p-type doping in in trans-polyacetylene for example, the oxidation of the system by the dopant produces a cation and a free radical electron on the material, and the charge neutrality is maintained by the formation of an anion on the reduced dopant.If this process occurs twice the two radical anions can recombine to form what is known as a spinless soliton, a positive or negative charge carrier free to independently move around the polymer chain. The process of forming solitons is unique to trans-polyacetylene due to it containing a doubly degenerate ground state which allows the soliton charges to move independently of each other (Figure 13).
Figure 13.Soliton formation using iodine as the dopant trans-polyacetylene. Reported with permission from ref.41.
However, most of the conjugated structures with applications in modern optoelectronic devices do not exhibit the same symmetry as trans-polyacetylene and therefore do not contain the degenerate ground state necessary for independent solitons to act as charge carriers. In this case, during the p-doping an electron is removed from the material and the cation and radical are again formed; the resulting cation and radical electron are partially delocalized over the π-conjugated system and is termed a polaron. An illustrative example of p-doping in polythiophene with BF4- as dopant is
reported in Figure 14. The polaron induces the formation of the quinoidal structure leading to a distortion in bond lengths over the section of the material linking the radical and cation. This distortion is not energetically favoured and is therefore usually stabilized ov er more than one repeating unit. Upon further oxidation, which can be induced by increasing dopant concentration, the unpaired radical electron of the polaron can be removed which leads to a sequence of quinoid type monomer units along the polymer which are separated from the normal aromatic monomer units by the two cationic charges. These sections of the material are kno wnas bipolarons and at high dopant levels are responsible for charge transport across the structure.46
Figure 14.On the left: example of polaron and bipolaron formation in polythiophene upon doping with BF4
concentrations of the strong electron acceptor F4TCNQ. Adapted with permission from ref.40 and ref.41;© The
Author(s) 2017.
With this approach the conductivity of conjugated materials can be tuned over several orders of magnitudeas shown in Figure 14 for the p-doping of P3HT.Today, molecular doping enables the high performance of modern benchmark organic devices by concurrently reducing ohmic losses in charge transport layers and injection barriers at interfaces to electrodes.47
1.1.5. Optical Properties of OSCs
Organic semiconducting materials possess fascinating optical properties and they have been extensively investigated and exploited for many applications associated with light interactions. Conjugated structures are able to absorb photons and may have many vibronic possibilities depending on the structural dipoles, which can give rise to anisotropy and several distinct absorption bands.48
Interacting with light, highly ordered materials generally exhibit sharp absorption bands as sociated with different vibronic features. On the other hand, amorphous materials generally display broad and smoothed Gaussian absorption distribution due to the disorder in the film or in solution (Figure 15).
Figure 15. Absorption spectra of ordered and disordered films of a polythiophene derivative. The
inset is the chemical structure of the derivative. Source: Reprinted Figure 1 with permission from T. Kobayashi et al., Phys. Rev. B 62, 8580. Copyright (2000) The American Physical
Specifically, the absorption of a photon of appropriate energy promotes the excitation of an electron from the HOMO to the LUMO in the absorbing molecule leading to the formation of an electron -hole pair system, also known as exciton. As previously discussed, excitons can be formed photonically or electrically and they can diffuse through the system via hopping charge transport mechanism.36 In the
Figure 16. a) Exciton formation upon photoexcitation and (b) exciton formation upon electroexcitation. Copyright © 2018 Morgan & Claypool Publishers
Organic semiconductors generally have high absorption coefficients at the peak of their absorption spectrum and, as such, the exciton generation rate can be maximised by increasing the overlap between the absorption profile and the source spectrum. According to their spatial extent and binding energy, excitons can be divided in (a) small, tightly bound excitons referred to as Frenkel excitons, and (b) large and loosely bound excitons referred to as Wannier–Mott excitons.49Large or small, in
this context it is related to the lattice constant. Exciton transport is governed by diffusion processes, with transport directed away from regions of high exciton concentration. The lack of molecular interaction in organic semiconductors influences exciton transport, which o ccurs through a hopping mechanism as already discussed. Excitons can move across the lattice through either dipole –dipole interactions (Förster resonance energy transfer) or wave function overlap (Dexter energy transfer); recombine radiatively (emitting a photon) or nonradiatively; or separate into free carriers.50,51 Almost
all organic materials have relatively high (higher than the thermal energy) exciton -binding energies, and this is of fundamental importance for, for example, OPV applications where optical excitations need to be separated into free carriers.
Atomic movement influences the relative positions of the atoms in a molecule and this affects the electronic states that absorb and emit light. In disordered materials, there are usually too many small variations to be identified, but in ordered systems there are particular vibrational modes that can even dominate the optical properties.48 Since both absorption and emission favourably occurs from the
vibronic state with the lowest energy, the absorption and photoluminescence spectra should be, in the ideal case, mirrored. Actually, since several defects are typically present in the solid-state and some of the vibronic transitions may be forbidden for, for example, symmetry reasons, the absorption and emission spectra are in reality rather dissimilar. The exciton migration toward a lower energy level also means that ordered domains will have a bigger impact on emission than absorption. Absorption and emission from the lowest vibronic level do usually not take place at the same energy despite the same states being involved. The reason for this is the reorganization energy. Both the energy of the excited electron and that of the resulting hole are shifted into the energy gap by one reorganization energy (Figure 17).48
In short, this means that the emission is shifted by two reorganization energies associated to the absorption. The disorder in the structure, and the fact that there are many distinct electronic states that can absorb light in a solid, usually make it difficult or impossible to identify the appropriate transitions, and thus the reorganization energy, at room temperature. The qualitative correlation remains, however, and materials with a large so-called Stokes shift (i.e., a large red shift of the emission spectrum compared to the absorption spectrum) tend to have large reorganization energies. As already mentioned, excitons can recombine radiatively o r non-radiatively. The rate of exciton recombination depends, among other things, on the spatial overlap of the electronic wavefunctions of the constituent hole and electron. Variations in electronegativity along the structure due to molecular asymmetry can cause alterations in the electron and hole charge densities and thus increase the exciton lifetime. Excitons can also be an intermediate in recombination processes. Electrons are spin -1/2 particles, that is, fermions, and the total spin of the exciton c an therefore be either 1 or 0. No two fermions can occupy the same quantum state. Excitons with a total spin of 0 are called singlet excitons and excitons with a total spin of 1 triplet excitons. Photoexcitation typically generates singlets, which can recombine quickly and radiatively (fluorescence). In a random population, such as in an electroluminescent device, however, there will also be triplets. Mathematically, the ratio between singlets and triplets is 1:3. Triplets are more long lived and do not rec ombine radiatively in organic semiconductors. Without intersystem crossing, for example, elec troluminescent devices are thus limited to a quantum efficiency of at most 25%.52 Hence, metal–organic molecules capable of
emitting light from triplet decay (phosphorescence) are ty pically used together with fluorescent organic semiconductors in OLEDs.53 However,room-temperature phosphorescence emission is
observed not only for transition metal complexes, but also for so me metal-free molecular crystals, where intramolecular motions are suppressed by several inte rmolecular interactions in the crystal lattice, thereby minimizing nonradiative decay from the electronically excited triplet state. The search for room-temperature phosphorescent organic compounds in the crystalline state and the establishment of guidelines for designing room-temperature phosphorescent organic molecules, along with crystal engineering, have been of crucial research for efficient electroluminescent devices.
Figure 18. Population scheme of singlet and triplet levels of organic materials after electro- or photo- excitation. For fluorescent emitter materials, the internal quantum efficiency (ηint) can be maximum 25%. For
phosphorescent emitter materials, the singlet excitons created are efficiently transferred to the triplet state via intersystem crossing (ISC) and the internal quantum efficiency can ideally reach 100%.
recombination rate is still generally much faster than the dissociation rate at reasonab le electric fields. To solve this, it is possible to use two-phase systems, where there is a suitable energetic offset between the respective FMOs of the two materials (Figure 19). When one of the materials is excited, either the hole or the electron will transfer to the other phase while still being electrostatically tightly bound. Such a state is called a charge transfer state and the corresponding exciton a charge transfer exciton. The electron and hole of a charge transfer exciton becomes (even more) spatially separated and the wave function overlap reduced, which results in an increased lifetime that is proportional to the frontier orbital offset. Many material combinations can reach close to 100% charge separation efficiency in this way.54
Figure 19. Sketch of light absorption–induced charge transfer exciton formation either by formation of an exciton on the donor molecule followed by transfer of the excited electron into the acceptor molecule LUMO (solid arrows) or direct formation where an electron from the donor HOMO is excited into the acceptor LUMO.
1.2. Devices: “The Big Three”
Among the different organic (opto)electronic devices, this section deals with “The Big Three”, i.e. OFETs, OLEDs and OPVs, and mainly focuses on their working principles and the corresponding functional materials typically used. As discussed so far, the electronic and optical properties of a given material are affected by film order, and film order is to a large extent controlled by the chemical structure of the molecule or polymer as well as the film-forming process conditions. In general, the choice of which material is most suitable for a chosen application depends on many aspects, which makes design and screening of materials for a chosen application a remarkably complex and multivariate task. It is hard to predict the functionality of a material in a device solely on the basis of the properties of the isolated system, thus the “perfect” material for a given application is an ambitious target that requires significant trial-and-error work.48
1.2.1. Large Area Electronics with Organic Field-Effect
Transistors
conductor, semiconductor, and dielectric materials. Organic materials are attracting attention for cost-effective and large-area pervasive electronics applications. Organic thin‐film transistors (OTFTs) can be fabricated with printing technologies on arbitrary substrates, enabling both high -throughput and low-cost production (Figure 20). In particular, the most widely used are the organic “field‐effect” transistors (OFETs) which exploit an electric field to modulate the conduction of a channel located at the interface between a dielectric and a semiconductor. The first OFETs based solely on organic materials had field-effect charge-carrier mobilities (µFET) of less than 10-2 cm2V-1s-1 and were still
vastly inferior compared to amorphous silicon (a-Si) MOSFET (µFET = 10–1 cm2V-1s-1). Nevertheless,
after 10 years of intensive research solution-processed OFET components are now approaching the field mobilities of amorphous silicon (maximum mobilities µFET of 0.6 cm2V-1s-1).56 In OFETs,
solution-processable active materials allow cost-effective and large-area processing in roll-to-roll methods, for example, for the electronic control of large active-matrix displays or in electronic labels. Moreover, the avoidance of slow and cost-intensive vapor deposition methods in high vacuum brings cost advantages. For instance, the use of OFET in electronic labels, so-called RFID tags (radio frequency identification tags), requires a production cost of less than one cent per label.57
Figure 20. Roll-to-roll production of flexible integrated circuits.
The structure of an OTFT generally consists of five parts: a source (S) electrode, a drain (D) electrode, a gate (G) electrode, an organic semiconductor (OS) layer, and a dielectric layer, deposited on a substrate (that could be a flexible plastic or paper). The most common OFET device configuration contains a thin film of the organic semiconductor deposited on top of a dielectric with an underlying gate electrode (Figure 21).Charge-injecting source-drain (S-D) electrodes providing the contacts are defined either on top of the organic film (top-contact configuration) or on the surface of the FET substrate prior to deposition of the semiconductor film (bottom-contact configuration).Other device structures are shown in Figure 21.38
